The present invention relates to a method of producing bleached wood pulp in which wood chips are digested and are then subjected to subsequent bleaching stages that are conducted in the presence of a sodium hydroxide. More particularly the present invention relates to such a method that includes an ozone bleaching stage in which a waste stream produced from an ozone bleaching stage is scrubbed to produce an oxygen containing stream useful in an oxygen delignification stage of the pulping process. Even more particularly, the present invention relates to such a method in which the waste stream is scrubbed by white liquor either in an external stage or during production of polysulfide liquor and then is separately reacted with white liquor to produce oxidized white liquor containing an appreciable amount of thiosulfate species of sulfur (thiosulfate liquor), and fully oxidized white liquor, containing almost no thiosulfate sulfur, to serve as sodium hydroxide in bleaching stages.
In the formation of bleached wood pulp, wood chips are digested in the presence of white liquor, which contains sodium sulfide and sodium hydroxide for such digestion, to produce brownstock pulp and weak black liquor. It is known that pulping with polysulfide liquor has advantages over conventional white liquor cooking in the wood chip digestion stage. The brownstock pulp is then washed and weak black liquor is extracted for reprocessing. The pulp is then subjected to oxygen delignification. The oxygen delignification is conducted in the presence of thiosulfate liquor, oxygen and steam. After the oxygen delignification, the wood pulp is sequentially subjected to an ozone bleaching stage, an extractive oxidation stage, which may be conducted in the presence of peroxide, and a final peroxide or chlorine dioxide bleaching stage. The extractive oxidation stage is conducted in the presence of thiosulfate liquor. Fully oxidized white liquor is a sodium hydroxide source for peroxide based bleaching stages and has advantages in such bleaching stages over thiosulfate liquor.
The ozone feed to the ozone bleaching stage is made in an ozone generator from air or more preferably oxygen. The end result is a mixture of ozone and oxygen containing about 5% ozone if air is used and anywhere from 10 to 14% ozone if the ozone is generated from oxygen. Not all of the feed to the ozone bleaching stage is consumed and as a result, a waste stream is produced that contains ozone, oxygen, carbon dioxide and water. This waste stream is further processed by an ozone destruct unit and a carbon dioxide scrubber to produce oxygen that can be used in an oxygen deliqnification stage. Ozone is destroyed so that the stream may be recycled to the ozone generator after CO.sub.2 removal and drying. Also, some of the waste stream may be vented atmosphere and ozone must be destroyed for industrial hygienic reasons. Carbon dioxide must be removed, otherwise it would consume sodium hydroxide inside the oxygen delignification stage, limiting the extent of lignin removal.
As will be discussed, the present invention provides a method of producing bleached wood pulp in which a waste stream produced from an ozone bleaching stage is scrubbed and then used as a source of oxygen for oxygen delignification. Expensive ozone destruct units are not used and in fact oxygen requirements can be balanced with oxygen recovery from the waste stream. The implication of this is that an oxygen recycle involved in the utilization of the ozone destruct unit can be eliminated together with its attendant capital and power consumption. Additionally, there is no need to further purify the waste stream to remove carbon dioxide and water. Moreover, the present invention advantageously utilizes polysulfide liquor in the wood chip digestion stage, oxidized white liquor in oxygen delignification and extractive oxidation stages, and fully oxidized white liquor in the peroxide bleaching stage.